1. Technical Field
The present invention relates to a surface polishing machine, and more particularly to, a surface polishing machine for machining a highly flat surface on disk-shaped workpieces such as semiconductor wafers, wafers mounted with semiconductor circuits, and magnetic disks, or plate-like workpieces such as glass substrates.
2. Related Arts
In conventional surface polishing implemented by a rotating an annular polishing tool, the size of a workpiece is naturally limited. Specifically, the polishing can be accomplished only for such a workpiece having a diameter smaller than a radial width of an annular polishing tool face of the polishing tool, the radial width being measured as the distance between two points located on the same diametral line of and crossing the inner and outer peripheries of the annular polishing tool face, respectively.
In a conventional arrangement exemplarily shown in FIGS. 6 and 7, an annular polishing tool 102 having a predetermined thickness is placed on a turntable 101. The outer diameter D0 of the annular polishing tool 102 is the same as that of the turntable. Symbol d0 denotes the inner diameter of the annular polishing tool 101, i.e., the diameter of a central through hole formed therein. Symbol W denotes the radial width of the annular polishing tool face 102A of the polishing tool 102. In the arrangement shown in FIG. 7, the polishing tool face width W is represented by equation W=(D0-d0)/2.
Reference numeral 100 represents a disk-shaped workpiece placed on the polishing face 102A. This disciform workpiece 100, serving as an object of polishing, must have its diameter D1 smaller than the width W of the annular polishing tool face of the polishing tool 102.
The workpiece 100 is always kept pressed toward the annular polishing tool 102 by a pressure holding plate 201 having the same diameter as that of the workpiece, whereby the workpiece is always applied with an appropriate pressurizing force during the polishing. In this example of FIGS. 6 and 7, the plate 201 has the same diameter K0 as that of the workpiece 100.
The pressure holding plate 201 is permitted to rotate and move in unison with the workpiece 100, while pressurizing the workpiece 100 toward the polishing tool 102, as the polishing tool 102 rotates. Reference numeral 202 denotes a polishing-position retaining mechanism for setting the pressure holding plate 201 and the workpiece 100 at a predetermined position on the polishing tool face 102A and for retaining them at that position.
Upon start of polishing of the workpiece 100, the turntable 101 is driven by a drive motor, not shown, and rotates in the direction of arrow R. Thus, the workpiece 100 placed on the polishing tool face 102A attempts to make a rotary motion in unison with the polishing tool 102 but is retained at the position shown in FIG. 7 since such a rotary motion is prohibited by the polishing-position retaining mechanism 202.
At the same time, due to a difference between the peripheral velocities of the polishing tool 102 on the inner- and outer-peripheral sides, the workpiece rotates around its axis on the polishing tool face 102A in the direction of arrow C shown in FIG. 7, while being pressed by the pressure holding plate 201. For this reason, an abrading action occurs between the workpiece and the polishing tool face 102A, whereby the workpiece 100 is polished uniformly accurately by the polishing tool 102.
As the number of times of usage of the polishing tool increases, the polishing tool is wear out, and irregularities and surface roughening of the tool are caused. In this respect, attempts have been made to make dressing with use of a dresser to remove constituent material of the polishing tool from its face, to create a smooth polishing tool face, so that the polishing with adequate evenness or flatness may be achieved.
As disclosed in Japanese patent KOKAI publication no. 7-130688, another attempt is made to alternately repeatedly carry out polishing and washing processes to ensure that the polishing is always done in a clean condition.
During polishing, moreover, abrasive fluid is normally supplied to the polishing tool face by dropping the same on part of the polishing tool face or by spraying it over the entirety thereof.
In the above-mentioned prior art example, the workpiece is required to have a diameter K0 smaller than the radial width W of the annular polishing tool face 102A of the polishing tool 102. Thus, the prior art entails a drawback that the annular polishing tool 102 becomes significantly large in diameter, as the diameter of the disciform workpiece 100 becomes larger.
If the polishing is carried out in a state where the workpiece 100 is retained at substantially the same position on the polishing tool, the polishing is accomplished by using only a corresponding part of the polishing tool face. In this case, the polishing tool face is susceptible to scratching at that part.
In case that a disk-shaped dresser is utilized to create a flat polishing tool face so as to carry out the surface polishing with satisfactory evenness, a problem is caused such that only part of the polishing tool face 102A can be stripped off since the diameter of the disk-shaped dresser is, in most cases, smaller than the width of the polishing tool face.
As mentioned above, the disciform workpiece 100 is sometimes subject to polishing and washing processes which are alternately repeatedly done with the intention of always implementing the polishing in a clean condition. However, this technique entails a problem that an increased number of steps are required and much time is needed for the polishing.
If the abrasive fluid is dropped on part of the polishing tool face 102A or if it is sprayed over the entirety of the tool face, the abrasive fluid tends to congregate at part of the tool face or to be dispersed to the surroundings of the tool face or dispersed into the atmospheric air. This prevents efficient supply of the abrasive fluid to the tool face.
Another type of surface polishing machine is also known in which the polishing of semiconductor wafers, magnetic disks, or the like is implemented with use of a disciform polishing tool instead of an annular polishing tool, as described in Japanese patent KOKAI publication nos. 4-33336, 5-69310, and 5-309559.
As exemplarily shown in FIG. 11, a conventional surface polishing machine of this type is provided with a disciform polishing tool 541 adapted to be rotatively driven and having a machining face thereof directed upward. For the surface polishing, a workpiece 542 is in contact with the upper face (polishing tool face) of the polishing tool 541 under pressure of a pressure holding plate 543 while abrasive fluid 544 is supplied to the polishing tool face.
The above-mentioned conventional polishing machine poses a problem that the evenness in workpiece thickness becomes worsened since an amount of removal of material at an inner peripheral portion of the workpiece is smaller than that removed at an outer peripheral portion thereof, so that the inner peripheral portion becomes thicker than the outer peripheral portion, as shown in FIG. 12. This problem is caused by two major factors: First, the abrasive fluid flowing into a central part of the machined face of the workpiece is insufficient in quantity, and secondly, a central part of polishing tool face, which is in contact with a center part of the workpiece face, is subject to significant wear and gouge, as compared to other portions of the polishing tool face.